selftests/bpf: Test __arena pointer members

Add a struct_ops member to bpf_testmod whose argument struct publishes
arena pointers: a plain one, a __nullable one that is left NULL, one
behind a nested struct, one inside a union, next to a scalar, together
with a kfunc that fills the struct with the kernel address of an __arena
argument and calls the member.

Two callbacks read the pointers back. One shares the header with the
module, so the members are arena pointers on the program side as well
and need no cast; it also records the register a load produces, which
must be the arena offset with the upper 32 bits clear, reads the member
with a load-acquire as well, which has to yield the same pointer, and
hands the pointer to an __arena kfunc. The other mirrors the vmlinux.h
rendering of the struct, a kernel pointer with the type tag as an
attribute, and casts the members to the arena address space the way a
program built against vmlinux.h has to. Both increment the value through
each pointer and the trigger checks the count. The first callback also
reads a member through a plain pointer to a bad address, a probed load
that is skipped or faults, and checks that it reads as NULL rather than
as an arena pointer.

The failure cases cover a store to the member, which a struct_ops
program cannot do to any argument member, a load from the union, an
exchange, one insn loading the arena pointer on one path and a scalar or
a memory pointer on another, and a program without an arena. On a
kernel whose module BTF carries no type tags the member tests are
reported as skipped.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019132203.3034698-10-memxor@gmail.com

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